Trophic Level Changes and Alewife Predation on Conesus Lake

Abstract

Conesus Lake has gone through a series of changes within its trophic levels. Walleye have been declining in numbers since the early 1970's. The introduction of an obligate planktivore, the alewife, in the late 1970's resulted in a rapidly expanding planktivore population and poorer water quality. Attempts at biomanipulation were begun in 1985 with yearly stockings of 65,000 3-5 cm walleye fingerlings. While these fingerling plantings were not successful, nevertheless alewife in Conesus Lake declined by 41% from 1985 to 1988. A likely cause for this decline is a decrease in the alewife's forage base. In 1988, this study was initiated to evaluate the effect of biomanipulation on water quality and zooplankton community structure. The expected shift to a larger zooplankton community was not observed. The weighted mean length of zooplankton declined from a value of 0.60 mm in 1972-73 to 0.23 mm in 1985 down to 0.18 mm by 1988. Ninety-eight percent of the 1988 zooplankton were less than 0.35 mm in size and ninety-three percent of the crustacean zooplankton were less than 0.35 mm. Zooplankton biomass in 1988 decreased to one-half the biomass of 1985. The zooplankton community changed from a Rotifera-Eucopepoda (calanoids and cyclopoids) -Cladcocera community in 1985 to a Rotifera-Cladocera Eucopepoda (cyclopoids) assemblage by 1988. The disappearance of Diaptomus and appearance of Eucyclops agilis and Macrocyclops albidus in the pelagic waters of Conesus Lake were unexpected. Water quality did not improve as expected. Turbidity, chlorophyll a, pH and soluble reactive phosphorous increased significantly (P < 0.022) within the epilimnion in 1988 compared to 1985.